Varying Time Intervals between New Moon and Full Moon, Why?

[Anton Viola]

I inspected time intervals between the different moon phases, for instance May 2024. It turns out that the duration between the New Moon and Full Moon was 15.44 days, still the whole lunar cycle lasted 29.39 days, so the large time interval between New Moon and Full Moon was "compensated" by a smaller interval between Full Moon and the next New Moon (29.39 - 15.44 = 13.95 days).

What is the cause of this, can someone explain?

Thanks!

Anton at www.astronomy-morsels.ch

 

 

Edited May 25 by Cornelius Varley
text colour corrected

[vlaiv]

Imagine two situations.

In first situation we observe difference in half of sidereal period and half of synodic period of the moon:

So moon starts at "0 degrees" with respect to earth and after half of lunar month it will be at "180 degrees" - but that is not yet completely changed phase from new to full. It needs to go a bit more time to catch up with earth and the fact it rotated around the sun. While moon is catching up - earth is moving itself so moon has to catch up a bit more.

Both motions are in same direction and it takes a bit longer for moon to catch up than if earth was just stationary at this position.

Now we look at other half of the cycle:

We again start at "neutral", but this time moon is on the other side and we once again compare sidereal to synodic.

After half of sidereal period - moon still needs to catch up with the fact that earth orbited around the sun - but this time as it moves to catch up - earth is moving in opposite direction - shortening the distance that moon needs to catch up as it is catching up.

This means that first part of synodic cycle is longer that second part of synodic cycle - simply because "it is more longer" than first half of sidereal cycle than second part of synodic cycle is longer than second half of sidereal cycle. Both parts of sidereal cycle are the same - but with synodic - in first half of it - moon will race to catch up earth that is moving away - while in second part, moon will race to catch up earth that is moving to meat it.